Typical power substation circuit breakers are used to open or close 3-phase power lines to energize or de-energize a city block or bigger load. The utility companies service the circuit breakers, but before they can put them back in service, they need to insure that the circuit breakers perform within their specified operating parameters. One such operating parameter is the contact opening and closing time (in milliseconds).
Before the utility personnel can even touch the circuit breaker, they must take the circuit breaker off the transmission line (by opening the circuit breaker) and isolating it. One of the safety requirements is to put safety grounds on both sides of the circuit breaker contact. These safety grounds eliminate any static discharge in the circuit breaker contact so that the personnel will not get shocked when they touch the circuit breaker. Additionally, these grounds are used to conduct any line power close to the circuit breaker while the personnel are working. The ground will take the line power to ground and create a fault (over current) on the transmission line. In this case, the protection system will de-energize the entire circuit and hopefully save the crew.
Current methods of testing substation circuit breakers generally involve testing an “off-line” circuit breaker by applying a voltage or current source to the circuit breaker contacts, electrically energizing the breaker, and measuring the voltage and/or current at the circuit breaker terminals. Open, close, open-close, close-open, or open-close-open timing characteristics can then be derived based upon the timing of the voltages and currents at the circuit breaker terminals with respect to the initiation sequence. An open pair of circuit breaker contacts is indicated by the presence of voltage and the absence of current at the circuit breaker terminals, while a closed pair of circuit breaker contacts will be indicated by the absence of voltage and the presence of current at the circuit breaker terminals.
However, safety practices currently in place in many substation environments require that each terminal of the circuit breaker be physically connected to earth ground with a low-resistance shunt while the circuit breaker is not in use (i.e. “offline”). These earth grounds present a short-circuit condition to the traditional test equipment, and therefore the circuit breaker always looks to be in the closed state. In practice, it is necessary to remove at least one of the safety grounds from the circuit breaker prior to testing, and to reinstall the ground(s) after testing is complete. This procedure is not desirable as it introduces a possibility that the ground may not be reconnected after the test, presenting a safety hazard.
Therefore there is a need for a system that tests substation circuit breakers while the circuit breakers are grounded on both sides providing increased safety.